Accessing SecurityPolicy SupportingToken in STS TokenProvider

I would like the AuthnStatement of an issued SAML2 assertion to contain an
AuthNContextClassRef which reflects the SupportingToken specified in the
SecurityPolicy binding protecting the STS Issue operation. So I am wondering
how I could access the SupportingToken state from within my TokenProvider. I
was thinking something along the lines of:
PhaseInterceptorChain.getCurrentMessage().get(Message.IN_INTERCEPTORS)
And then running through the interceptors to get the associated handlers,
with the aim of getting at the WSSecurityTokenHolder.

But is nothing more than a (poorly) educated guess. I am hoping for some
guidance...

Thanks

Dirk